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Technique Precisely Predicts Color from White OLED

Photonics.comApr 2013
EINDHOVEN, Netherlands, April 15, 2013 — A technique that precisely calculates the color of light produced from white OLEDs could significantly improve OLED design processes and reduce the cost.

Regarded as the light sources of the future, OLEDs are flexible and transparent light-emitting surfaces made of very low cost materials. White OLEDs consist of stacked, ultrathin layers, each emitting its own color, and all together resulting in white light. Until recently, it was not possible to predict the exact color produced by a white OLED; manufacturers had to rely on trial and error.

To predict what kind of light an OLED design will produce, researchers at Eindhoven University of Technology and Philips Research, both in the Netherlands, Dresden University of Technology in Germany and other institutes developed computer models of the complex electronic processes in OLEDs on a molecular scale. These showed, for example, the injection of electrical charge, the creation and distribution of the excitons, and the creation from these of individual photons.

A transparent OLED made at Philips Research Aachen, seen from the rear; light is emitted from the front. Researchers at Eindhoven University of Technology and other institutes have developed a tool that allows the light color of OLEDs to be predicted precisely. Courtesy of Eindhoven University of Technology/Bart van Overbeeke.
“At first we thought it would never be possible,” said Peter Bobbert, a researcher at Eindhoven University of Technology. The main difficulty was that each change in the electrical charge also influences all the other charges, which makes the simulation extremely complex.

The investigators overcame the obstacle using Monte Carlo simulations — computational algorithms that rely on repeated random sampling to obtain numerical results — with nanosecond steps. They can now predict where light is produced and lost in the ultrathin layers, making it possible to optimize OLEDs so they produce the same amount of light using less electric power. Results from Eindhoven correspond to measurements carried out at Philips on OLEDs made at Dresden University of Technology.

The scientists expect that the efficiency can still be increased by a factor of three. Manufacturers also can use the findings to design OLEDs with specific colors. They can calculate in advance exactly how thick the different layers need to be, and how much pigment has to be added to the layers. The much shorter and less costly design process will allow the overall development costs to be reduced, leading to lower prices for the final products.

“This has already been possible for a long time in the field of microelectronics, with the ability to precisely predict the behavior of integrated circuits,” Bobbert said. “Now we can do the same thing with OLEDs.”

The research — funded by the European Union (FP7 project AEVIOM), the Dutch Polymer Institute, NanoNextNL and NanoNed — was published online in Nature Materials (doi: 10.1038/nmat3622).